1. Field of the Invention
This invention relates to a multi-network system, by which a plurality of stations communicate data to each other through a plurality of transmission lines and in particular to a multi-network system suitable to transmission from a bridge station connecting two different transmission lines.
2. Description of Related Art
Recently needs to transmit data among a plurality of terminals in the precincts or between two terminals which are in different precincts.
Now the number of terminals connected to one network system increases and if all the terminals were connected through one transmission line, transmission throughput would decrease. In order to prevent the decrease of the transmission throughput, it is necessary to construct a multi-network system, by which terminals are divided into plural groups and made belong to a plurality of transmission lines, which are in turn connected to each other.
In a multi-network system two different transmission lines are connected by a bridge station. The bridge station receives from a transmission line a data frame destined for the other and sends it to an adjacent transmission line. In this case, when the transmission speed, the frame type or the transmission access method is different for different transmission lines, the bridge station sends the data after having effected a transformation treatment for the transmission speed, the frame type, etc. by means of a buffer register incorporated in the bridge station.
As a prior art multi-network system a method is proposed, by which each of the ordinary stations, which are not bridge station, stores the address of the bridge stations connected to the same transmission line, each of the bridge stations storing the address of all the stations connected to two transmission lines, with which it is connected; in the case where a certain station sends data to another station connected to a transmission line other than the transmission line, with which the former is connected, in the data frame the destination address, which is the address of the station receiving the data, is the address of the bridge station and the source address is the address of the former station itself; the address of the station, which should receive finally the data, is included in the information portion in the data frame and send together; and the bridge station interpretes the information portion of the received data frame, recognizes that it is a transmission destined for another transmission line, changes the destination address to the address of the station, which should receive finally the data or in the case where the data should be transmitted further through another bridge station, to the address of the bridge station, and sends the data frame, keeping the source address, which is the original source address, or changing it to the own address of the bridge station. According to this method, in the case where data are transmitted between two stations connected to different transmission lines, there are problems that each of the bridge stations on the data transmission course must read transmitted information, interprete the final destination station and modify the destination address and the source address, that treatment time for this interpretation and address transformation, and that as the consequence the transmission throughput cannot be increased. Furthermore there are problems that each of the ordinary stations must store the address of the bridge stations so that it can have the address of a bridge station in the case of the relay between different transmission lines and that each of the bridge stations must store the address of the all the stations connected to the transmission lines to which the bridge station itself is connected in order that it can be judged whether the destination station is connected with the transmission line to which the bridge station is connected or not.
As another prior art multi-network system, in Japanese Patent unexamined publication No. 84-62245, has been proposed a method, by which the address code of the transmission line, to which the station which should receive the data belongs, is contained in the transmission data frame; in the case where the data transmission is effected in a same transmission line, the address code of the transmission line is set to a predetermined value; and in the case where the data transmission is effected between two different transmission lines, the bridge station receives only data frames, whose address code of transmission line has no specified value and sends the data to the other adjacent transmission line, while changing the address code to a predetermined value. However, also according to this method, there are problems, in the same way as described above, that the bridge station must rewrite the transmission line address relating to the destination station and further that the bridge station must store the address of all the stations connected to the transmission line, to which the bridge station itself belongs.